After few more trials, finally concluded that the drone is caused by (harmonic) vibration. Looks like stock BMW muffler has a bolt-on damper. (#18303413587). Need tp find something similar that can be bolted on a "magic" spot in the exhaust somewhere (since I can't weld!). Those 2 rubber mounts may not be sufficient to handle the driving vibration since I have no drone when I load at P or N.